Publications

Issue Archive

Methods of Fabricating Scintillators With Radioisotopes for Beta Battery Applications

Friday, 01 February 2013

Applications for these power sources are implantable medical devices, power supplies for
remote monitoring, and “trickle chargers” for consumer applications.

Technology has been developed for a
class of self-contained, long-duration
power sources called beta batteries, which
harvest the energy contained in the
radioactive emissions from beta decay isotopes.
The new battery is a significant
improvement over the conventional phosphor/
solar cell concept for converting this
energy in three ways. First, the thin phosphor
is replaced with a thick scintillator
that is transparent to its own emissions. By
using a scintillator sufficiently thick to
completely stop all the beta particles, efficiency
is greatly improved. Second, since
the energy of the beta particles is absorbed
in the scintillator, the semiconductor photodetector
is shielded from radiation damage
that presently limits the performance
and lifetime of traditional phosphor converters.
Finally, instead of a thin film of
beta-emitting material, the isotopes are
incorporated into the entire volume of the
thick scintillator crystal allowing more
activity to be included in the converter
without self-absorption.

Scintillator-Based Beta Battery: Isotope containing scintillator and surrounding scintillator captures virtually all of the energy emitted by the beta emitter." class="caption" align="right">There is no chemical difference
between radioactive and stable strontium beta emitters such as Sr-90, so the beta emitter
can be uniformly distributed throughout a strontium
based scintillator crystal. When beta emitter
material is applied as a foil or thin film to the surface
of a solar cell or even to the surface of a scintillator,
much of the radiation escapes due to the
geometry, and some is absorbed within the layer
itself, leading to inefficient harvesting of the energy.
In contrast, if the emitting atoms are incorporated
within the scintillator, the geometry allows
for the capture and efficient conversion of the
energy of particles emitted in any direction. Any
gamma rays associated with secondary decays or
Bremsstrahlung photons may also be absorbed
within the scintillator, and converted to lower
energy photons, which will in turn be captured by
the photocell or photodiode.

Some energy will be lost in this two-stage
conversion process (high-energy particle to
low-energy photons to electric current). The
geometric advantage partially offsets this as
well, since the absorption depth of high-energy
beta radiation is much larger than the depth of
a p-n junction. Thus, in a p-n junction device,
much of the radiation is absorbed far away from the junction,
and the electron-hole pairs are not all effectively collected. In
contrast, with a transparent scintillator the radiation can be
converted to light in a larger volume, and all of the light can
be collected in the active region of the photodiode.

Finally, the new device is more practical because it can be
used at much higher power levels without unduly shortening its
lifetime. While the crystal structure of scintillators is also subject
to radiation damage, their performance is far more tolerant of
defects than that of semiconductor junctions. This allows the
scintillator-based approach to use both higher energy isotopes
and larger quantities of the isotopes. It is projected that this
technology has the potential to produce a radioisotope battery
with up to twice the efficiency of presently used systems.

This work was done by Noa M. Rensing, Michael R. Squillante,
Timothy C. Tiernan, William Higgins, and Urmila Shirwadkar of
Radiation Monitoring Devices, Inc. for Glenn Research Center.

Question of the Week

This week's Question: Last week, Elon Musk, chief executive of Tesla, said that the electric car maker would introduce autonomous technology, an autopilot mode, by this summer; the technology will allow drivers to have their vehicles take control...